This invention relates to an electronic analog filter for smoothing the stepped output from a digital-to-analog converter.
In many applications having a sampled data signal processed in digital form, it is necessary to convert the digital signal to analog form. Typically, this is when a digital computer is required to produce an analog output. There are numerous potential applications in industrial process control systems, aerospace control systems, automotive control systems, and the like.
The computer, or other type of digital processing system, produces an output which is a continuous series of values in binary form as though each value were a sample of a continuous function, where the samples are taken at equally spaced intervals called the frame time, or clock period where the clock rate is established by the computer. The output is held constant at each value for one frame time (clock period), after which it rapidly switches to the next value. Each value is converted from digital to analog form virtually instantaneously, so the analog output is a stepped waveform.
A stepped waveform output of this kind is unacceptable for many applications. One method of remedying this situation is to increase the sample clock rate, i.e., decrease the frame time for each value, but this method is usually not available since it decreases the time available for computing the value. Only a very high speed computer with few computation steps per value can use this method. The more usual method involves the use of an effective analog filter, thereby increasing the amount of computation that can be used to compute each value in one frame time.
For a smooth filtering operation, a band-pass RLC filter would be required having a high Q factor, which is a figure of merit that measures the relationship between the energy stored by the inductance, L, and the capacitance, C, and the rate of dissipation of energy. In its simplest form, an RLC filter is comprised of a resistor in series with an LC tank circuit. Its Q factor is given by the ratio of its reactance in the tank circuit to its effective series resistance at a given frequency. Since the frequency of the signal being smoothed is a function of the digital values, and may vary from virtually zero (for a steady state digital output) to some upper limit, which is a function of the dynamic response of the load receiving the digital-to-analog converted signal being smoothed, the bandwidth of the filter is apt to be quite large.
To facilitate design of a band-pass filter, it has been the practice to use operational amplifiers coupled to simulate inductance as disclosed in U.S. Pat. No. 3,835,399. This permits the design of a stable tunable band-pass filter capable of operation over a wide range of band widths and center frequencies. Such an active filter would help meet the high Q requirements of a low-pass filter. But such an active filter is not itself adequate for the task at hand due to the presence of the sample frequency and the attendant phase angle of the filter over a certain range of frequencies.